This invention relates generally to screening machines of the type used to separate or classify mixtures of particles of different sizes. More particularly, the invention relates to a mechanism and method for supporting and moving a screen deck within the screening machine.
In screening machines of the type described, a screen is mounted in what is often called a xe2x80x9cscreen deckxe2x80x9d which includes a supporting peripheral frame around the perimeter of the screen. The screen deck may include one or more screens and the screen(s) may be woven, an aperture plate or another design. Typically associated with this screen deck are other material handling elements which are moved with the screen and form walls or partitions above or below the screen for containing the liquid and/or particulate materials adjacent to the screen and directing them to appropriate outlets. These elements may comprise a top cover and a pan beneath the screen. In the case of screening machines with multiple screens or deck units, spacer pans or frames are provided between the multiple screens.
The screen deck is releasably mounted to a carrier, frame, table or box to which vibratory motion is imparted, typically by one or more eccentric motors or other means of excitation. The carrier, frame, table or box and associated screen deck are referred to herein as a xe2x80x9cscreen boxxe2x80x9d. The screen box may be moved in oscillatory, vibratory, gyratory, gyratory reciprocating, fully gyratory, rotary or another type of motion or combinations thereof, all of which are herein collectively referred to as xe2x80x9cvibratoryxe2x80x9d motion or variations of that term.
Screening machines of this type are used for a wide variety of applications. For example, in a plastics factory, a granular material is often produced as an intermediate product prior to being processed into the end product. The plastic granulate particles are often referred to as xe2x80x9cpelletsxe2x80x9d and have a specific size and shape. The uniformity of the pellets make it possible to accurately meter the quantity to be fed to the machine which produces the end product. The precise metering is very important for reducing the amount of waste. Therefore, the pellet manufacturer attempts to produce granular material which is absolutely uniform with regard to shape and size. However, in many cases and for various reasons, defective pellets are unavoidable, even if it is only in small quantities. This defective granular material can be of various types. For example, a number of individual pellets may be stuck together (three or four particle groups, etc.), pellets which are much longer than a desired size (over lengths or xe2x80x9clongsxe2x80x9d) or pellet strands in which the individual pellets are partly but not completely cut (bead chains) are commonly manufactured. The irregular pellets to be screened and separated from acceptable particles can have a wide variety of shapes (straight particles with many times the length of normal particles such as the longs, twisted partly cut strands, etc.). All these defective pellets make accurate metering impossible, and therefore lead to a significant waste quantity for final processing.
The problem of prevention or subsequent separation of waste particles not only occurs in the production of plastic pellets but is also encountered wherever a heap of uniformly shaped and sized particles or material is to be produced by a corresponding manufacturing process, e.g. molding, compacting or cutting, whereby they can also be an end product and not an intermediate product as is the case with plastics (e.g. activated carbon pressed articles, granulated foodstuffs, etc.).
Since defective granulate particles cannot generally be prevented in the various manufacturing processes, the problem of optimum separation of these waste particles which, if not separated, signify a reduction in the value of the finished or intermediate product, exists.
Prior attempts to separate the waste particles by screening processes of various types have been only marginally successful. Commonly, prior methods have failed because the longs or straight shaped particles corresponding to the desired cross-section, but having many times the desired length, are placed on end by the screening movement and then drop vertically through the sieve opening, i.e. are not held back. One source of such problems in the screening operations is vertical or movement of the screen in a direction perpendicular to the screen. Movement of the screen in such an out-of-plane direction tends to up-end the pellets and provides them the opportunity for proper orientation to pass through the screen. As such, longs or long pellets are not separated out.
A less intense screening movement, i.e. a lower amplitude of vibration admittedly prevents the long, straight shaped pellets from being vertically positioned, but the screen surface then becomes clogged within a short period of operation by near-mesh sized material or defective granulate. The longer that the defective particles remain atop the screen, the more likely they will have an opportunity to pass through or clog the screen. The partly cut, three-dimensionally curved strands (bead chains) also hook themselves into the sieve openings, making it necessary to manually remove such defective granulate every so often. These problems also occur if oblong perforated plates or long mesh sieve netting are used. The use of step sieves (step perforated plates) does not ensure the trouble-free separation of defective granular material from the heap.
The problem is therefore to find an improved method and associated system which reliably and cost-effectively permits the separation of waste particles, minimizes the time the defective pellets are on the screen, and reduces, if not eliminates, out-of-plane or vertical movement of the screen.
The above-described and other problems with prior art screening machines and associated methods have been resolved by this invention. Presently preferred embodiments of this invention include a screening machine with a fixed base and a perforate screen deck mounted for movement relative to the base during a screening operation. The screen deck includes a head end, a tail end and a longitudinal axis extending between the head and tail ends. An inlet is provided to discharge particulate matter to be screened onto the perforate screen deck near the head end. A pair of outlets are located near the tail end of the screen deck, with one of the outlets discharging defective particulate matter that remains atop the perforate screen deck and the other outlet discharging acceptable particulate matter that passes through the perforate screen deck. A collection pan is located below the screen deck to collect the particulate material that passes through the screen deck prior to being discharged through the second outlet.
A drive motor is coupled to the screen deck near the head end to impart two-dimensional vibratory motion within a plane to the screen deck to promote separation of the particulate material by the screen deck. The vibratory motion is defined in part by longitudinal movement of the screen deck in a direction generally parallel with the longitudinal axis toward and away from the head end.
In accordance with one aspect of the present invention, a mounting assembly for the screen deck includes at least one leaf spring connecting the tail end of the screen deck to the fixed base so that the leaf spring supports the tail end of screen deck for movement relative to the base. The leaf spring(s) are oriented horizontally to avoid introducing vertical movement to the screen deck as it moves to and from the head and tail ends. Since the leaf spring(s) are oriented horizontally, the only motion of the screen deck at the tail or discharge end is longitudinally, general linear motion while the motion at the head end is vibratory motion within the two-dimensional plane.
In one preferred embodiment, each outer end of each leaf spring is bolted or otherwise secured to a mounting bracket affixed to one of the side walls of the screen deck. Each such mounting bracket includes a plate that extends parallel to the side wall of the screen deck and a tab that extends outwardly from the plate and away from the side wall. The inner central portion of each leaf spring is bolted or otherwise secured to a tab extending upwardly from the base of the machine. Preferably, the tab is fixed and does not move, deflect or bend during operation of the machine so that the screen deck moves in a longitudinal direction, i.e., between the head and tail ends, during a screening operation.
In another embodiment, the mounting assembly includes preferably four leaf springs, but at least two leaf springs. The springs are assembled into a spring pack for supporting the tail end of the screen deck on the base. The springs in the leaf pack are generally parallel, and oriented perpendicular to the longitudinal axis of the screen deck when the springs are not flexed. The central portion of the two inner springs are secured to the tab extending upwardly from the base. The central portions of the two outer springs are each secured to the bottom of the screen deck. The corresponding outer ends of all the leaf springs are secured to each other in a spaced arrangement to maintain the parallel relationship of the unflexed springs. One of the principle advantages of this mounting assembly is that the significant tension and compression forces of the flexing springs are not transferred to the base and the screen deck. The generally symmetrical design of the spring pack provides support for all the loads present, including the weight of the screen deck as well as bending/flexing and twisting/torsional motion of the springs throughout the stroke of the screen deck. As a result, stress on the mounting brackets is reduced to likewise reduce fatigue failures possible with other designs.
In accordance with another aspect of the present invention, the leaf springs are offset or deflected when the machine is in the neutral mid-point of its stroke. This arrangement advantageously utilizes the resulting bias of the leaf springs to accelerate, assist or enhance movement of the screen deck during portions of its stroke and likewise to decelerate, hinder or retard the movement during other portions of its stroke. In one embodiment, the leaf springs are initially deflected or offset by spacers positioned between the outer ends of the springs and the tabs extending outwardly from the mounting brackets. Alternatively, the springs can be mounted to the screen deck in an offset, flexed or deflected position.
With an appropriate offset, the leaf springs promote a slow advance of the screen deck toward the tail end and a quicker return movement toward the head end that works on the inertia of the material being processed on the screen deck. With such an arrangement, the time that the longs or defective pellets are on top of the screen deck is reduced, thereby minimizing the opportunities for them to pass through the screen deck along with the properly sized pellets. The biased arrangement of the leaf springs also increases the effectiveness of the screening process (i.e., fewer defective pellets passing through the screen deck) while decreasing the screening time. The amount of offset, and the direction of the offset, is determined by the application, machine specifications or other parameters to achieve desired results.
Therefore, according to this invention, the effectiveness of the screening operation is increased and the speed with which the screening operation is accomplished is likewise increased due in large part to the leaf spring mounting arrangement for the screen deck and the biased loading of the springs relative to the stroke of the screen deck.